The objectives of this research program are to examine the psychophysiological correlates of infant temperament and to investigate the consequences of specific patterns of affective bias on social behavior and peer interaction in school age children. A variety of data suggest that the pattern of EEG asymmetry recorded during a resting state may be a marker for certain temperamental styles. In general, subjects with a pattern of greater relative right hemisphere activation are more likely to respond with negative affect or dysphoric mood state in response to mild stress or novelty. This predisposition to respond with negative affect nay, if it is stable through early childhood, have important consequences for psychological well-being of children and adolescents. In an attempt to investigate the early origins of this pattern of brain asymmetry and its consequences for later social functioning two studies are proposed. The first study will screen 400 four month old infants to select those who are on the extreme of motor reactivity and affective bias. Four groups will be selected: infants with high motor reactivity and a predisposition to positive affect; infants with high motor reactivity and a predisposition to negative affect; infants with low motor reactivity and a predisposition to positive affect and infants with low motor reactivity and a predisposition to negative affect. These four groups will be followed in a longitudinal study in which their responses to a set of standard affect elicitors will be monitored. In addition, brain electrical activity will be recorded at each assessment. Infants will be seen at 9, 14, and 24 months of age. The relations between infant temperament and individual patterns of EEG asymmetry across this period will be explored. A second study will examine the relations between EEG asymmetry and social behavior in a group of randomly selected, non-referred, four year old children. These children will be assessed interacting with their peers and individually for their affective bias. Two assessments are planned: one at age four and a second at age seven. The stability of social behaviors and individual patterns of EEG asymmetry and their interrelations across the three year age span will be assessed. This research will be among the first to explore the relations between patterns of brain electrical activity and social behaviors in infants and young children and may yield important data on the psychophysiology of early social development.